Add code to GlobalVariable.h so that global variables marked as

externally_initialized return false for hasDefiniteInitializer and
hasUniqueInitializer.

rdar://12580965.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174345 91177308-0d34-0410-b5e6-96231b3b80d8